For many polyolefin applications, including films and fibers, increased melt strength and good optical properties are desirable attributes. A higher melt strength allows fabricators to run their blown film lines at a faster rate. It also allows them to handle thicker films in applications such as geomembranes.
Typical metallocene catalyzed polyethylenes (mPE) are somewhat more difficult to process than low-density polyethylenes (LDPE) made in a high-pressure polymerization process. Generally, mPEs (which tend to have narrow molecular weight distributions and low levels of branching) require more motor power and produce higher extruder pressures to match the extrusion rate of LDPEs. Typical mPEs also have lower melt strength which, for example, adversely affects bubble stability during blown film extrusion, and they are prone to melt fracture at commercial shear rates. On the other hand, mPEs exhibit superior physical properties as compared to LDPEs. In the past, various levels of LDPE have been blended with the mPE to increase melt strength, to increase shear sensitivity, i.e., to increase flow at commercial shear rates in extruders; and to reduce the tendency to melt fracture. However, these blends generally have poor mechanical properties as compared with neat mPE. It has been a challenge to improve mPEs processability without sacrificing physical properties.
U.S. Patent Application Publication No. 2007/0260016 discloses blends of linear low density polyethylene copolymers with other linear low density polyethylenes or very low density, low density, medium density, high density, and differentiated polyethylenes, as well as articles produced therefrom.
U.S. Pat. No. 6,300,451 discloses ethylene/butene/1,9-decadiene copolymers, and ethylene hexene vinyl norbornene copolymers (see Tables I and II). The decadiene terpolymers disclosed are designed to be used alone and not in blends for improved processability/property balance. The relatively high MI of the resins suggests that they would not be suitable in blends which exhibit improved extensional strain hardening.
Patil, et al., “Rheology of Polyethylenes with Novel Branching Topology Synthesized by Chain Walking Catalyst,” Macromolecules, 2005, 38, pp. 10571-10579 discloses dendritic PE produced from chain walking catalyst. The dendritic PE prepared by chain walking catalysts has extensive short and long chain branches with combined branch density of greater than 100 branches per 1000 carbon. The extensive short chain branching leads to amorphous polymers which have limited use in mixtures with semicrystalline polyethylene resins of commercial interest. Additionally, these polymers are prepared at low temperatures and extremely low pressures, both conditions that are not commercially attractive. Additionally, blends are not disclosed in this paper and there is no mention of blown film compositions.
Ye, et al., “Chain-Topology-Controlled Hyperbranched Polyethylene as Effective Polymer Processing Aid (PPA) For Extrusion of a Metallocene Linear-Low-Density Polyethylene (mLLDPE),” J. Rheol., 2008, 52, pp. 243-260 discloses that the processability of Exceed™ 1018 Polyethylene, in terms of melt fracture, could be improved with an addition of the hyperbranched PEs made from chain walking polymerization at more than 3 wt %. Because the hyperbranched PE is immiscible with mLLDPE, it was speculated that the hyperbranched PE forms phase-separated droplets, which can migrate to the die surface and form a lubricating layer promoting extrudate slippage.
U.S. Pat. No. 6,870,010 discloses blown films with improved optical properties produced from blends of linear metallocene PE with high MW HDPE. While the optical properties as measured by haze are improved over unblended film composition, the mechanical properties as measured by Dart Impact suffer a significant deterioration.
U.S. Patent Application Publication No. 2011/0118420 discloses dendritic hydrocarbon polymer and process for the production thereof. U.S. Patent Application Publication No. 2011/0118420 also states in the background section, that “ . . . [w]hile LCB technology has been a part of the polyethylene industry since the 1930's, there is still a need to further optimize the type and availability of LCB polyethylenes and other polymers. A useful, inexpensive blend additive in the form of a LCB polymer could significant[ly] impact the processing/performance balance for polyethylenes, particularly the multi-billion dollar market for polyethylene films and molded articles. There could be even greater use in polypropylene, where there is currently little commercially viable technology for incorporating LCB. There is also a need for LCB polymers in the EPDM elastomer market.”
Other references of interest include: Guzman, et al. AIChE Journal May 2010, vol. 56, No 5, pg. 1325-1333; U.S. Pat. Nos. 5,670,595; 6,509,431; 6,870,010; 7,687,580; 6,355,757; 6,391,998; 6,417,281; 6,114,457; 6,734,265; and 6,147,180.
We have discovered that certain branched hydrocarbon modifiers will advantageously improve processability of polyethylene without significantly impacting its mechanical properties. Moreover, addition of these branched hydrocarbon modifiers provides a means to change such properties on a continuous scale, based on real-time needs, which is typically not possible due to the availability of only discrete polyethylene grades. Furthermore, a different set of relationships between processability and properties is obtained, compared to those available from traditional polyethylenes and their blends with conventional LDPE, which allows for new and advantageous properties of the fabricated articles.
More particularly, the present invention relates to polyethylene compositions having improved properties such as melt strength or extensional strain hardening, without substantial loss in blown film, dart impact, MD tear, or other mechanical properties. Additionally, the films produced from these compositions exhibit surprisingly excellent optical properties as measured by lower film haze.
Further, this invention relates to polyethylene compositions having improved bubble stability. Previous attempts to remedy the situation by addition of long-chain-branched PEs such as LDPE or other branched PEs (U.S. Pat. No. 6,870,010) have resulted in decreased mechanical properties. Some of the blown films blended with branched PE additives additionally suffered from poor optical properties, e.g., the existence of gel particles. There is an industry wide need to find modifiers that improve processability without loss in mechanical properties, and more preferably with enhancement in one or more mechanical properties.
The current invention solves the problem by using a crystalline long-chain-branched hyper branched polyethylene (disclosed in U.S. patent application Ser. No. 13/302,446) that is effective in improving processability at very low concentration levels (1%). The films produced from the 1% blend of this crystalline long-chain-branched hyper branched PE with Exceed™ 2018CA PE exhibit the following combination of properties, among other things, which have not been possible in the prior art: a) free of gel and excellent optical properties; b) improved film processability in terms of bubble stability, film gauge variation; and c) enhanced MD/TD tear and no loss in most of the mechanical properties except impact strength.